In the manufacture of disposable pens for various types of ink jet printers, various approaches have been taken to insure that a substantially constant backpressure is provided in the ink reservoir of the pen as the ink is depleted from full to empty during a printing operation. In this manner, the size of the ink drops ejected from an orifice plate of the pen will remain constant during ink depletion, and additionally this constant backpressure will prevent leakage of ink from the orifice plate when the pen is inactive. One such approach to providing a substantially constant backpressure in the ink reservoir of a thermal ink jet pen is disclosed and claimed in the U.S. Pat. No. 4,509,062 issued to Robert Low et al and entitled "Ink Reservoir With Essentially Constant Negative Backpressure".
Whereas the approach described in the above Low et al patent has proven highly satisfactory and unique in many respects, this approach nevertheless requires a collapsible bladder in order to maintain a substantially constant backpressure in the ink reservoir over a certain range of ink depletion therein. This requirement for a collapsible bladder has certain attendant disadvantages which are overcome by the present invention and will be appreciated and better understood from the description to follow.
Another prior approach to providing a controlled backpressure in the ink reservoir of a different, stencil type of pen utilizes a so called capillary compensating technique wherein the main ink reservoir of the pen is connected to a capillary ink flow path or groove. This path or groove is operative to receive a varying quantity of ink during ink reservoir depletion to thereby maintain a substantially constant back pressure in the main ink reservoir. One such capillary compensating technique used in a stencil pen is disclosed in German Patent No. 2,844,886 issued to Witz et al.
Whereas the above technique in the German Patent No. 2,844,886 may be suitable over a limited ink reservoir volume and a limited range of operating temperatures, the stencil pen of the above German Patent is not capable of handling relatively large ink reservoir volumes operating over relatively large changes in ink operating temperature. Furthermore, the capillary groove capacity of the pen disclosed in the above German Patent No. 2,844,886 will typically be on the order of eight to ten percent of the ink reservoir capacity, and this ratio in turn means a relatively large increase in capillary groove capacity for desired corresponding increases in ink reservoir capacity. Thus, this eight to ten percent volume of capillary groove requirement in the Witz et al German patent imposes a rather substantial limitation on pen construction where a significant increase in size of the ink reservoir of the pen is required.
Another recent approach to providing a controlled backpressure in an ink reservoir of a disposable ink jet pen is disclosed and claimed in copending application Ser. No. 880,774 of Jeffrey Baker et al, filed July 1, 1986, assigned to the present assignee and incorporated herein by reference. In this latter approach, a reticulated polyurethane foam is used as an ink storage medium for both black and color ink jet pens. This more recent technique of storing ink in a porous medium such as polyurethane foam provides several new and useful improvements and distinct advantages with respect to the earlier bladder storage techniques. However, the requirement for a porous foam storage medium in the ink storage compartment of the pen limits the volumeric ink storage efficiency thereof.